Make Guild.default_role use get_role.
Make Guild.default_role use get_role.

Prevents some stale caching by using the new O(1) Guild.get_role.

imayhaveborkedit/discord.py,rapptz/discord.py,Harmon758/discord.py,Harmon758/discord.py,khazhyk/discord.py,Rapptz/discord.py